FAQs for Vets

What are the clinical signs?

Primary stage infection

The clinical course of FIV infection depends on many factors including the dose and strain of the infecting isolate and the age and health status of the infected cat. In experimentally infected kittens, clinical signs may be seen during the primary stage of virus replication, usually limited to those of a vague malaise. Primary stage disease is rarely identified in naturally infected cats in the field.

Secondary stage infection

Secondary stage disease, characterised by profound lymphoid depletion and severe immunodeficiency, has been described. Stomatitis, cytopenias, chronic or recurrent infections, wasting and neurological problems may all be associated with secondary stage FIV infection. Many clinical presentations have been associated with FIV infection in the field and, although it is often difficult to show direct associations, an increased risk of lymphosarcoma has been demonstrated in FIV-infected cats. Because of the difficulty of disease staging, sick FIV-infected cats should not be assumed to be suffering from the effects of FIV infection, particularly in areas where FIV is prevalent. It is important to stress that many FIV-positive cats have treatable diseases.

How is FIV infection diagnosed?

Serological tests

Since recovery from FIV infection has not been recorded, the detection of antibodies against FIV is used as a marker for infection. In-house ELISA and immunochromatography tests detect anti-FIV antibodies and diagnostic laboratories may use immunofluorescence or Western blotting to detect antibodies. Western blots allow the detection of antibodies to individual FIV proteins and may be used for confirmation of positive serological tests.

Both ELISA and immunochromatography tests are generally appropriate in most situations. However, these tests have limitations because the diagnostic specificity is less than 100% which is especially important in low prevalence populations and when healthy cats test positive. For example, an FIV prevalence of 1% results in one positive test per 100 cats and a diagnostic specificity of 99% also results in one false positive in the same 100 cats. This gives two positive results in 100 cats, only one of which is correct (hence a positive predictive value equal to only 50%). Any positive result in a low prevalence population (e.g. young, indoor, pure bred cats) must therefore be confirmed, for example using Western blots. A positive result in a cat from a high-risk group (e.g. a free roaming, aged, male cat) is likely to be a true positive because the frequency of true positives will exceed the frequency of false positives in this population. In addition, negative results in low prevalence populations are generally very accurate, with the following exceptions. Firstly, false negative results may be obtained early in infection, when cats become provirus positive but remain seronegative for several weeks to months. Secondly, false negative results may be obtained in the terminal stages of disease when high levels of virus may lead to sequestration of anti-FIV antibodies in virus-antibody complexes.

PCR assays

Polymerase chain reaction (PCR)-based assays that detect proviral DNA, are available for the diagnosis of FIV infection. However, it has been shown that such PCR tests are variable in performance and may even be inferior to serological tests, with sensitivities and specificities ranging from 40 to 100%. Strain variation is an important factor in the reliability of PCR assays and may account for discrepant results when identical samples are sent to different laboratories. Discrepant results may occur when serology and PCR are compared. A sample may test seropositive but PCR negative; this result may be explained by the presence of an FIV subtype not recognised by the PCR, rather than by the absence of FIV infection. Alternatively samples may test seronegative but PCR positive; such an outcome may be observed where a cat is living in close contact with FIV-infected seropositive cats can become provirus positive without developing detectable levels of serum antibodies or disease. Such cats are infected and the majority will seroconvert weeks to months later.

Virus Isolation

A highly reliable method of diagnosis is virus isolation. Peripheral blood lymphocytes are prepared from fresh samples of heparinised blood and are co-cultivated with primary feline T cells for 2-3 weeks; the presence of virus in cultures is confirmed by measuring the levels of viral core proteins in the culture fluids. This procedure is laborious and is used mainly for research purposes.

Can false positive results arise?

Kittens born to FIV-infected queens may test seropositive in the absence of infection as a result of passively acquired maternal antibodies. In such cases, kittens should be retested after approximately 16 weeks of age, by which time (in most cases) levels of maternal antibodies will have declined to undetectable levels so that a positive result is indicative of FIV infection in the kitten. However, in rare cases antibodies may persist up to six months and so a kitten testing seropositive at 16 weeks-of-age should be retested two months later. If it is still positive at six months of age, the kitten is infected. If an earlier result is required, PCR may be employed to detect virus negative kittens, but in such cases it is important that the queen is tested in parallel to ensure that the PCR test employed will detect the infecting strain.

How is FIV transmitted?

The majority of natural FIV infections are acquired by biting, presumably through the inoculation of virus, or virus-infected cells, from the saliva of persistently infected cats. Transmission from mother to kittens may occur, but not all of the offspring become persistently infected. The proportion of kittens infected depends on the level of virus circulating in the queen during pregnancy and birth. For example, if the queen is acutely infected then up to 70% of the kittens may be infected. However, if the queen is clinically normal but chronically infected, few kittens will be infected.

Although neither oronasal nor venereal spread has been documented in the field, cats can be infected by experimental inoculation of virus into the nose, mouth, vagina and rectum and virus can be recovered from semen following natural or experimental infection. Queens may, however, be infected at mating if bitten by an infected tom cat.

How can we prevent infection?

Epidemiological studies suggest that the major natural mode of transmission of FIV is via the inoculation of virus in saliva during biting. The virus survives only briefly outside the host and so indirect transmission is unlikely. Therefore, FIV-positive cats do not need to be quarantined when they are hospitalised or boarded. Routine hygienic measures to prevent the spread of FHV, FCV and FPV will be sufficient to prevent horizontal transmission of FIV. Similarly, standard sterilisation procedures for surgical and dental equipment will readily inactivate FIV. Indirect transmission may be more significant in multicat households. Ideally, FIV-infected and uninfected cats should be housed separately. Where this is not possible, careful attention should be paid to separating and cleaning feeding bowls. FIV-infected cats, particularly those with oral inflammation, shed virus in saliva that may be a source of infection for uninfected cats if food bowls are shared. Separation of litter trays is also advised. Where FIV-infected and uninfected cats are housed together, owners should be encouraged to keep a closed colony. Maintaining a stable group will prevent exposure of further uninfected cats and will also reduce the stress and aggression that may be associated with new arrivals.

The only sure way to prevent an adult cat from becoming infected with FIV is to isolate uninfected cats. Confining FIV-infected owned cats is likely to be insignificant in controlling the spread of infection cats allowed to roam outdoors since the prevalence of FIV in feral cat populations is high. Neutering of pet cats is strongly advised for many reasons. Neutering might be expected to reduce roaming and territorial aggression, thereby decreasing the likelihood of FIV infection.

FIV-infected cats should not be used for breeding. Cats being used as blood donors should be tested for FIV and maintained indoors. If these cats have previously been allowed to roam, a repeat test 2-3 months after the first is advisable to detect cats that were infected but had not seroconverted prior to the first test.

What is the prognosis for FIV-infected cats?

The life expectancy of naturally infected cats or the proportion that develop second stage disease is unknown.

How should FIV-infected cats be managed?

Routine health care is important in FIV-infected cats. Stress reduction by avoiding overcrowding and observing good husbandry practices is advised. A good quality commercial diet should be fed. Control of ectoparasites, endoparasites and the importance of dental prophylaxis should be stressed to owners. Veterinary health checks should be scheduled every six months. This should include a full physical examination to facilitate the early detection of any emerging health problems.

Asymtomatic cats

ABCD recommends that cats should never be euthanased solely because of an FIV positive test result. Similar to HIV infection in human beings, there is a long latent period during which FIV-infected cats appear healthy with no clinical signs. This “asymptomatic” period of infection may last for several years and during this time cats may have normal happy lives.

However, FIV-positive cats have an increased likelihood of developing clinical signs, mainly due to secondary infection, immune-mediated disease or neoplasia. The duration of the “asymptomatic” period varies according to the infecting isolate. Based on experimental studies, it is thought that cats infected at a younger age are more likely to progress to an immunodeficiency state.

During the 6 monthly veterinary health check, the weight should be monitored and testing for haematology and biochemistry should be considered. CD4 and CD8 monitoring to stage FIV infected cats is controversial and is neither generally available nor realistic in most practice situations.

Surgery is generally well tolerated by asymptomatic FIV-infected cats, but peri-operative antibiotic administration should be used in all surgeries and dental procedures. FIV-infected cats can be housed in the same ward as other hospitalized patients; they should, however, be housed in individual cages and kept apart from cats with other infectious diseases. Under no circumstances should FIV-infected cats be placed in a "contagious ward" with cats suffering from infections such as viral respiratory disease.

Appropriate supportive treatment of FIV-infected cats relevant to the presenting clinical signs should be instituted as early as possible. If FIV-infected cats are sick, prompt and accurate identification of the secondary illness is essential to allow early therapeutic intervention and a successful outcome of treatment. Therefore, more intensive diagnostic testing should proceed earlier in the course of illness than might be recommended for uninfected cats. Many cats with FIV infection respond as well as uninfected cats to appropriate medications although a longer or more aggressive course of therapy (e.g., antibiotics) may be needed.

Symptomatic cats

There is limited information available on prognosis for FIV-infected symptomatic cats. It is important to stress that, especially where FIV-infection is common, the finding that a cat is seropositive for FIV during an investigation does not necessarily mean that the FIV-infection is responsible for all of the clinical signs. However, there is no doubt that a proportion of FIV-infected cats develop second stage disease. Chronic, recurrent and refractory problems are typically described. Clinical findings that are most often reported in FIV-positive cats include gingivitis, stomatitis, weight loss, chronic URT signs, chronic GI disease, renal disease, neoplasia and neurological problems. Cytopenias (neutropenia, lymphopenia and anaemia) are common in second stage disease. Immune-mediated diseases may also accompany second stage FIV infection.

Currently, the ability to stage disease progression in FIV-infected cats is very limited. Laboratory data used for disease-staging in HIV-infected humans, such as CD4 counts and viral load determination, are not commercially available for cats. There is insufficient data on the clinical course of FIV-infection in naturally infected cats to assume that extrapolations from the clinical course of HIV will be valid.

Clinically sick, FIV-infected cats should be investigated promptly and treated aggressively. Where antibiotics are indicated the choice should be based on the results of culture and susceptibility testing wherever possible and bactericidal agents are preferred to bacteriostatic products in potentially immunosuppressed cats.

Studies have shown that FIV status is not prognostic for response to chemotherapy in cats with lymphosarcoma. Another study showed that FIV-positive cats with cryptococcosis could respond well to treatment for cryptococcosis although FIV-positive cats generally required treatment for a longer period.

Immunosuppressed owners

Potentially immunosuppressed human patients, e.g. transplant recipients, should not have contact with FIV-infected cats because of the potential for shared opportunistic pathogens such as poxviruses, cryptosporidium and toxoplasmosis.

When is euthanasia necessary?

Generally euthanasia is not necessary until the end stages of disease.

Should FIV-infected cats be vaccinated with the core vaccines?

There are limited studies on the benefits of vaccination for FIV-infected cats. FIV-infected cats have been shown to be capable of making immune responses to some vaccines and routine vaccination should be practiced. Killed products are recommended since modified-live vaccines should be avoided in potentially immunosuppressed animals.

Is there an FIV vaccine?

At present there is no FIV vaccine available commercially in Europe. Experimentally, vaccine-induced protection against FIV infection has been achieved in cats using several immunogens, including inactivated virus or inactivated infected cell vaccines, canarypox-based vaccines in combination with inactivated cells and DNA vaccines. Of these vaccines, the most successful to date have been whole inactivated virus vaccines (WIV) preparations; one such vaccine has been available commercially to veterinarians in the USA since 2002, in Australia and New Zealand since 2004 and in Japan since 2008.

However, the efficacy of the FIV vaccine has not been tested against a range of European field isolates. In one study the FIV vaccine did not protect cats against a virulent UK primary isolate of FIV. Therefore, imported vaccinated cats might not be protected against natural challenge with the circulating European isolates of FIV. Furthermore, the FIV vaccine poses problems for the diagnosis of FIV infection since infected and uninfected vaccinated cats cannot be differentiated using serological tests.

ABCD does not recommend the use of the whole inactivated virus vaccine available outside Europe, given the problems associated with serological diagnosis of infections and lack of evidence of efficacy against European isolates.

Acknowledgements

The author gratefully acknowledges the contributions of her ABCD colleagues in the compilation of these notes.